Stainless steel by internal nitridation

ABSTRACT

An austenitic steel containing as a dispersoid therein particles of a metal nitride. The nitride particles have a free energy of formation of greater than about -21,000 cal/mole and are present at an interparticle spacing of less than about 10 microns.

United States Patent [191 Kindlimann [451. Apr. 16, 1974 STAINLESS STEEL BY INTERNAL NITRIDATION [75] Inventor: Lynn E. Kindlimann, Milford,

Conn.

[73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

[22] Filed: July 15, 1971 [21] Appl. No.: 163,084

Related US. Application Data [62] Division of Ser.- No. 735,186, June 7, 1968,

abandoned.

[52] US. Cl. 148/38, 148/16.6 [51] InLCl. C22c 39/14, C23c 11/16 [58] Field of Search l48/l2.1, 16, 16.6, 20.3,

9/1965 Vordahl 148/4 3,357,827 12/1967 Waeser et a1. 75/205 X 3,468,658 9/1969 Herold et al 75/122 FOREIGN PATENTS OR APPLICATIONS 526,777 6/1956 Canada 148/16.6

OTHER PUBLICATIONS Dispersion Strengthening of Iron Alloys by Internal Nitriding, Chen, August 1965, Transactions of AlME October 1952, pages 1083, and 1084.

Dispersion Strengthening of Iron Alloys by Internal Nitriding, Chen, August 1965, pgs. 54, 85.

Primary Exa miner-Charles N. Lovell Attorney, Agent, or Firm-Vincent G. Gioia; Robert F. Dropkin [57] ABSTRACT An austenitic steel containing as a dispersoid therein particles of a metal nitride. The nitride particles have a free energy of formation of greater than about 21,000 cal/mole and are present at an interparticle spacing of less than about 10 microns.

7 Claims, No Drawings 1 STAINLESS ISTEELBY INTERNAL NITRIDATION This application is a division of now abandoned copending application, Ser. No. 735,186, filed June 7, 1968.

Advances in the aerospace industry have brought about a substantial need for materials with improved high-temperature properties and capabilities. Many materials have been proposed to fill this need. The present invention provides austenitic chromium and/or nickel-containing steel, hereinafter referred to as austenitic stainless steel, with such improved high temperature strength and other properties as to render it particularly suitable for the requirements of the aerospace industry. Austenitic stainless steels in accordance with invention are particularly well adapted for use in service applications at temperatures above l,400 F. The properties of the steel in accordance with the invention are achieved by internally nitriding such steel to form, as a dispersoid, a nitride having a high free energy of formation. 7

Many techniques are known for dispersion strengthening of materials. This approach requires a two-phase structure consisting of hard particles dispersed in a strong but ductile matrix. Best properties are obtained when interparticle spacing (IPS) is small and the hard second phase is stable in the ductile matrix upon exposure for long periods at elevated temperatures. The most common technique used for dispersion strengthening is precipitation hardening wherein an unstable solid solution is held at a temperature at which there is sufficient atomic mobility for the phases of reduced solubility to precipitate on a fine scale from the matrix. However, such compositions cannot normally be used I for long periods of time above the precipitation temperature because of the possibility of resolution and/or growth of the precipitated particles with a resulting loss of strength. A second approach to dispersion strengthening involves the use of a refractory-like material such as thorium oxide (thoria) dispersed in a metal matrix such as nickel or a nickel alloy. Still a third approach has been through a technique involving internal oxidation of a metallic component, in this method, the material containing a strong oxide former is held in an oxy-,

gen-containing atmosphere that is reducing to the base material but not to the oxide former. Oxygen diffuses through the base material and reacts with the active element forming a relatively insoluble and finely dispersed hard phase throughout the matrix. These particles are. not as subject to the resolutioning problems mentioned above which are common in other precipitation hardening systems. However, the internal oxidation process has not found widespread commercial application primarily because of the very slow diffusivity of oxygen and the long time periods required for complete internal oxidation.

For. high temperature applications, austenitic stainless steels have traditionally been used because of their good corrosion'and oxidation resistance. However, no effective means for strengthening stainless steels other than through cold working have been known. Since the effects of cold working are rapidly lost at elevated temperature, strengthening by precipitation of of intermetallic compounds has also been considered. However, most of the alloys amenable to precipitation hardening are also susceptible to the resolutioning problems discussed above at temperatures in excess of 1,300 to l,400 F.

The present invention contemplates treating austenitic rather than ferritic steels by internal nitridation. The diffusion rate of the nitride components is less in austenite than in ferrite. Thus, austenitic dispersionstrengthened steels are capable of exposure to high temperature without experiencing particle growth. The austenite matrix is also stronger at high temperature.

In dispersion strengthening, it is desirable to have a sufficient volume fraction of dispersoid to reduce the interparticle spacing. However, it is not possible to achieve a greater number of particles by merely increasing the volume fraction of dispersoid. Moreover, because the dispersoids do not have a simple geometrical shape, there is no simple relationship between interparticle spacing, particle size and volume fraction of particles. It has been found, however, that the most important parameter to be controlled in the dispersion strengthening of alloys is the interparticle spacing referred to hereinafter as IPS. Although some strength improvement may be noted at an average IPS of 10 microns or greater, compositions within the scope of the invention contain dispersoids having an average IPS of about 10 microns or less, preferably less than 2 microns. It is possible by practicing the method of the invention to achieve IPS of below one micron. The average IPS as referred to herein is that value measured by the Line Fraction Method as described in the article Measurement of Particle Sizes in Opaque Bodies, by R. L. Fullman, Journal of Metals, March, 1953, page 447.

In accordance with the invention, a nitride dispersoid is formed within the steel by internal nitridation. Internal nitridation follows the laws of diffusion with the result that the time to completely nitride a given steel article is proportional to the square of the half thickness. This constitutes a limiting factor on the thickness for continuous in-line processing. In addition, the IPS increases with increasing depth from the surface.

Various nitride formers may be used within the purview of the invention. The requirements of such materials are that the nitride, i.e., dispersoid, have a sufficiently high free energy of formation to lead to the production of very small particles. Typically, such nitrides should possess a free energy of formation of greater than about "21,000 cal/mole. The nitride former should be present in an amount sufficient to provide an interparticle spacing of less than 10 microns and preferably less than 2 microns, the volume percent being dependent on the IPS, and being larger for a smaller IPS at a constant particle size. The nitride used should possess a very low solubility in the austenitic steel treated so as to possess a reduced tendency to coarsen at elevated temperatures such as those to which the dispersion-strengthened article will be subjected in use. Extensive evaluation has indicated that the preferred and by far most superior nitride former to be employed is titanium. Titanium has a relatively high solubility in stainless steel and its nitride has a very high free energy of formation. Other nitride formers are available, but none as good as titanium. For lower temperature applications, strengthening may be achieved by use of nitride formers such as aluminum, vanadium and columbium. Such dispersion-strengthened materials could be used at temperatures where coarsening is not too rapid, but these materials would not have the high temperature capabilities of titanium-dispersoid strengthened steel. Still other nitride formers such as boron, zirconium, cerium, hafnium, thorium, etc., are not soluble to a high extent in austenitic steel. In the preferred embodiment in which a titanium nitride dispersoid is formed, an austenitic steel containing 0.5 to 3 percent titanium is preferred. Less than about 0.5 percent titanium results in a product having satisfactory room temperature properties but the particles are such that they tend to grow and the resulting IPS, although under microns, would be relatively high. When more than about 3 percent titanium is present in the steel, additional improvement in properties may be obtained but such would be disproportionately less than obtainable with 0.5 to 3 percent titanium.

Since as noted above, titanium is regarded as vastly superior, the ensuing discussion and examples will be directed primarily to its utilization. It has been found that a given temperature, for a predetermined titanium level the more rapidly the nitriding is performed the smaller will be the resulting interparticle spacing. In addition, all things being equal, the IPS decreases as the temperatures decrease because of the greater number of particles nucleated. Since diffusivity decreases very rapidly with temperature, there are practical limits below which the process is not commercially economical. A high nitrogen gradient in the steel article is necessary to increase the nitriding speed at any given temperature. If another nitride such as that of chromium present in the stainless steel is also being formed during the treatment, then a measure of the nitriding speed for the material may be given by the equation below as represented by the symbol S.

S exp S ]erf S erf RS] C(% N,)

where exp S exponential S erf S error function of S (given in math tables) R ratio of the depth of precipitated Cr-nitrides to the depth of precipitated Ti-nitride at any time during the process N,= solubility of nitrogen in the base alloy without Ti at the temperature under consideration C a constant including the Ti level of the alloy It is seen from the equation that the higher the nitriding speed the faster the material can be nitrided and the smaller the resulting interparticle spacing for a given alloy at a given temperature since values of R (the ratio of the depth of precipitated chromium nitride to the depth of precipitated titanium nitride) greater than zero will increase the nitriding speed. There are practical limits on the ratio R since the speed of the advancing chromium nitride front is dependent upon the solubility of nitrogen in the alloy without either chromium or titanium. As long as the desired interparticle spacing is achieved the process will work efficiently regardless of how small the value for R and the nitriding speed. The smallest IPS' will generally be obtained in a given alloy at a given temperature by working at as high an R value as possible since this leads to the highest nitriding speed.

The present invention involves a considerable departure from conventional nitriding of metals. The latter normally involves treating in a temperature range of 900 to l,l00 F in flowing gaseous ammonia. When conventional nitriding is applied to a low alloy steel containing elements such as chromium, molybdenum,

titanium and aluminum, a very hard case results. The nitrided case is typically on the order of about 0.03 inch deep for normal -hour treatment at 930 F. The case hardness may vary between R 55 to 65. Conventional processing has been applied to stainless steels and, in particular, austenitic and ferritic steels, which after treatment show low resistance to galling so that nitriding of bearing surfaces, for example, is beneficial. The hardness of the nitrided case is produced by the precipitation of very fine nitrides of the reactive elements mentioned above, possibly in combination with each other and/or iron. In the case of stainless steel, the precipitates are chromium or chromium-rich nitrides. However, because of the removal of chromium from solid solution, the corrosion and oxidation resistance of case nitrided materials is greatly reduced. Moreover, the nitrided layer will soften on short exposure to temperatures above 1,200 F, probably due to dissolution of the nitride precipitate. Since conventional nitriding does not affect the interior of the material, there is a high edge-to-center nitrogen concentration gradient.

In practicing the invention to dispersion strengthen austenitic steels by internally nitriding, nitridation is performed under conditions different from those previously used for case nitriding. In practicing the invention the same mechanisms which cause softening, i.e., greater solubility and higher nitrogen diffusivity, in conventional nitriding and which are intentionally avoided are used with advantage to produce a material with dispersed nitrides. I have found that internal nitriding of austenitic steels may be accomplished at a temperature from about l,600 F to the melting point of the steel. Nitrogen may be supplied from an atmosphere which may be either properly treated ammonia or nitrogen, the latter is preferably, although not necessarily, pressurized, i.e., above atmospheric pressure. Mixtures of the two with each other and with other compatible gases may also be used. The term compatible gases as used herein refers to a non-oxidizing or inert gas such as hydrogen or argon. The lowest possible moisture content is desirable and ammonia substantially free of moisture and free oxygen is presently preferred. To achieve a nitriding rate in nitrogen gas at all comparable to the rate obtainable in ammonia, nitrogen should be under a pressure greater than atmospheric. The presence of small amounts of moisture or oxygen severely affects the nitriding rate in nitrogen. However, where it is desirable not to form chromium nitrides, the nitrogen or nitrogen-containing atmosphere may be used.

Austenitic steels containing chromium are internally nitrided in accordance with the invention of above l,600 F to avoid the problem of massive chromium nitride formation at the grain boundaries. Since the presence of chromium nitride is deleterious to oxidation and corrosion resistance, excess nitrogen over that necessary to react with the titanium present to form titanium nitride should be removed by proper atmosphere control. Substantial removal of excess nitrogen may be effected in a vacuum or by the use of a purging gas such as hydrogen or other gas non-reactive with the material. Homogenization of excess nitrogen in the metal can be accomplished in a mildly oxidizing atmosphere or in a diluted nitrogen containing atmosphere. Control of chromium nitride formation in a conventional hot wall furnace may also be achieved by adjusting the atmosphere, e.g., ammonia, flow rate. In a cold wall furnace, e.g., one where the workpiece is heated by induc- TABLE 11 tion, control of chromium nitride formation may be accomplished with the use of gaseous diluents added to code Trealmem the ammonia.

. 37 min. at 2400F in raw NH 211934 CFH To lllustrate the practice of the inventlon and the 1m 5 10 mm a 2200.}: in raw at 13 CF 1 at proved properttes obtainable thereby, reference 1s 2000F in H, p 39 min. at 2200F in raw NH at 9% CFH made to the following examples. In these examples, 20 min at 2300* in raw NHL" 13 CF sheet samples were nitrided and tensile tested both at room temperature and at 2,000F. The composition of the samples is described in Table I. As can be seen, 0 both samples were based on Type 304L steel with tita- Following nitriding as described above, the samples niumadditions. were tested for tensile properties and the tensile prop- TABLE 1 Composition of Test Materials Heat No. C Mn P S Si Cr Ni Ti 8 Fe RV2252 0.0060 0.48 0.008 0.003 0.75 18.01 12.00 1.22 0.010 Balance 111/2253 0.0094 0.32 0.010 0.005 0.72 18.01 12.00 2.03 0.010 Balance The samples described above were nitrided under erties are reported in Tables 111 and IV. The data in various conditions in ammonia. The nitriding condi- Table 111 are the results of tests at room temperature, tions are disclosed in Table I]. p 25 and Table IV reports the 2,000F tensile properties.

TABLE 111 Room Temperature Tensile Properties 0.2% YS UTS Elongation Code No. Sample ksi ksi in 2" %RA Control RV2252 25 74 60.0 41.5 Control RV2253 31 80 46.0 41.9 37 RV2252 63 122 22.5 23.2 37 RV2252-A 54 119 22.5 20.2 37 RV2253 132 21.0 32.2 37 RV2253-A 65.3 130 20.0 27.3 38-11 RV2252 79 136 20.0 26.7 38-H RV2253 89 142 26.8 39 RV2252 88 148 21.0 25.3 39 RV2252-A 94 147 11.5 15.8 39 RV2253-A 99 157 13.0 22.8 40 RV2252 132 18.0 25.7 40 RV2252-A 84 133 14.4 40 RV2253 82 143 12.0 30.0 40 RV2253-A as 144 18.0 25.1

"'A designates material was preannealed 5 minutes in air and pickled clean prior to nitriding. All other material was nitrided as cold worked. "Sample broke outside gage marks.

Samples were coated with the commercial product No-Carla" to protect against oxidation because of the thinness of the strip. "Two tests.

The effect of interparticle spacing on tensile properties is evident by comparing these factors. Interparticle spacing of the samples treated as described above are reported in Table V. It is apparent that the lower the IPS the stronger the material at 2,000 F.

TABLE V Interparticle Spacings UPS) Heat No. Code IPS The nitriding temperature used to produce the dispersoids is particularly important at greater thicknesses. Also the thickness itself influences the proper- 10 RV2252 37 2 tles obtalnable by mtemally mtndmg. These effects are RV2253 37 shown by the data presented in Tables VII and VIII RV2252 38-1-1 -.5 h th ff f (V2253 w 10 report e e ect o nltrl mg temperature on RV2252 39 1 room temperature mechamcal propert es and the effect Rv2253 39 of sheet thickness on room temperature mechanical Measured at 1 mil below the sample surface. p p respectlvely' TABLE VII Effect of Nitriding Temperature on Room Temperature Mechanical Properties 0 Nitriding Prior Sheet 0.2% YS UTS Elong. Temperature Condition Thickness (ksi) (ksi) 1V4 I800F Annealed 0.010" 59.8 79.9 3.0 2000F Annealed 0.010" 75.8 121.8 11.5 22151= Annealed 0.010" 63.7 123.3 18.0 19051= 50% CR. 0.002" 106.7 138.8 6.0 20001= 50% OR. 0.002" 102.5 134.2 5.0 1800F 50% 0.12. 0.005" 99.6 149.4 15.5 190sl= 50% 0.1a. 0.005" 98.5 144.7 11.5 2000F 50% C.R. 0.005" 98.5 146.6 15.0 I800F 50% C.R. 0.010" 87.0 138.8 14.0 1905F 50% C.R. 0.010" 87.0 137.3 12.5 2000F 50% C.R. 0.010" 8|.0 132.4 12.5

m ess.e uss -lm iMQHE lql flaw e 451m New e TABLE VIII Effect of Sheet Thickness on Room Temperature Mechanical The amount of tltamum present 1n the compos1t1on Properties has an effect on the yield strength of the material fol- Nmdmg Thlckness Temperature (ksl) (ksl) l /s" lowlng n1tr1d1ng. Thls will be seen by a serles of examples reported in Table VI. The test results reported in l800 99.6 149.4 15.5 0.010" I800F 87.0 138.8 14.0 the table are room temperature properues, but the 10 I380 60 strength at all temperatures wlll be reflected by the 0.005" 190s1= 98.5 144.7 11.5 properties reported. As discussed previously, it is preg-gg 33,: 5;? :3 ferred to treat an austenitic steel containing 0.5 to 3 1 146:6 percent titanium to obtain consistantly superior prop- 2000F 81.5 132.4 12.5 0.020" 20001= 75.8 122.5 10.5 M ertles. V I m Base material nominally l8Cr-l2Ni2Ti0.0lB-0.75Si0.SOMn-0.0IC All samples denitricled as in Table l. 0 7

TABLE VI Effect of Ti on Room Temperature Tensile Properties Sheet Nitriding 0.2% YS UTS 3i: Elong. Ti Thickness Temperature (ksi) (ksi) 1%" 0.78 0.005" I800F 72.7 103.0 12.5 2.03 0.005" I800F 99.6 149.4 15.5 0.78 0.010" I800F 65.0 115.8 32.5 2.03 0.010" I800F 87.0 138.8 14.0 0.78 0.005" 1905F 76.5 123.7 27.5 1.22 0.005" I905F 83.8 124.4 12.5 2.03 0.005" I905F 98.5 144.7 11.5 0.711 0.010" I905F 64.4 105.8 15.0 1.22 0.010" I905F 69.4 108.5 10.0 2.03 0.010" 1905F 117.0 137.3 12.5 0.78 0.005" 2000F 73.5 115.11 17.0 2.03 0.005" 2000F 98.5 146.6 15.0

Bale material nominally l8Cr-l2Ni-0.0l B-0.50Mn-0.75Si0.0lC Ti. Cold rolled prior to nitriding.

"All samples were denitrided 10 hrs. at 2000F in dry H, to remove soluble nitrogen.

TABLE [X Effect of Nitriding Temperature on 2000F Mechanical Properties of the 2.03% Ti Alloy (0.0l Thick) Nitriding Degassing 0.2% YS UTS Elong Temperature Time (ksi) (ksi) 1%" I905F 3 hrs. [3.8 24.0 3.2 22l5F 3 hrs. 13.5 19.4 7.2 l905F [0 hrs. l6.0 21.8 3.2 Z2l5F hrs. 12.3 17.8 9.2

*Degassed in dry H, at 2000F to remove excess nitrogen.

It is apparent from the above that changes and modifications may be made without departing from the invention. Thus, for example, if needed to provide particle stability during creep rupture, boron may be added. However, the presence of boron has been found to decrease the nitriding rate. In general, up to 0.01 percent boron may be satisfactorily employed. In addition, if desired in treating alloys of the l8-chromium, l2- nickel, 2-titanium type, the additional presence of as much as 2 percent silicon or 1 percent molybdenum has been observed to have no detrimental effect on the nitriding rate or mechanism. Carbon present has the effect of removing available titanium from solution. In many cases, it may be desirable to restrict carbon to a low level so as not to tie up titanium and avoid sensitization problems. Less than 0.03 percent carbon is desirable, but higher amounts are tolerable depending upon the intended application of the alloy. y

it has also been found that titanium contents of above about 2 percent not accompanied by increases in the nickel or manganese content may result in an austenitic-ferritic two-phased alloy. The presence of ferrite may ns se s tbsslt i rate, bu s s elsttitaaiym as titanium nitride makes the alloy fully austenitic.

As indicated above, it is highly desirable to degas following nitriding. However, the extent of degassing depends upon the intended application.

It is apparent from the above that various changes and modifications may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims.

l claim;

1. An internally nitrided aiisieiiifi gaiii' s eatsa taining chromium and a dispersoid therein of metal nitride particles having a free energy of formation of greater than 21 ,000 cal/mole, said metal nitride particles being of at least one metal from the group consisting of titanium, aluminum, vanadium and columbium, said nitride particles being present at an interparticle spacing of less than 10 microns and being essentially homogenously distributed throughout the said steel, said nitride particles increasing the high temperature strength of said steel, said steel being further characterized by being essentially devoid of chromium nitride.

2. Austenitic stainless steel according to claim 1 wherein said dispersoid is titanium nitride.

3. A composition according to claim 1 wherein said dispersoid particles are at an interparticle spacing of less than about 2 microns.

4. A composition according to claim 3 wherein said dispersoid particles are at an interparticle spacing of less than about 1 micron.

5. A composition according to claim 1 wherein said austenitic stainless steel contains nickel.

6. A composition according to claim 4 wherein said dispersoid is titanium nitride.

7. A composition according to claim 2 wherein said austenitic stainless steel contains from 0.5 to 3.0 percent titanium. 

2. Austenitic stainless steel according to claim 1 wherein said dispersoid is titanium nitride.
 3. A composition according to claim 1 wherein said dispersoid particles are at an interparticle spacing of less than about 2 MICRONS.
 4. A composition according to claim 3 wherein said dispersoid particles are at an interparticle spacing of less than about 1 micron.
 5. A composition according to claim 1 wherein said austenitic stainless steel contains nickel.
 6. A composition according to claim 4 wherein said dispersoid is titanium nitride.
 7. A composition according to claim 2 wherein said austenitic stainless steel contains from 0.5 to 3.0 percent titanium. 